The mother lays undeveloped eggs which are either fertilised outside the mother’s body (90% of bony fish reproduce this way), or inside, like some sharks and rays. The reproductive organs are paired and get bigger during the mating season. Ripe eggs (roe) pass out of the ovaries into the body cavity and can completely fill the space. The eggs leave the body through the urogenital opening.

Amongst fish which reproduce by ovopartity, there are several groups.

Egg Scatterers

Fish which breed in this way either spawn in pairs or in groups. Males and females release milt (the sperm and spermatic fluid of the male) and eggs into the water at the same time. These mix together, fertilising the eggs. The fertilised eggs are broadcast (or spread) into the plankton column and float away in the current or sink to the bottom. No parental care is given, so large amounts of eggs are produced. It is easy to produce many eggs and because they are in the water, they don’t dry out – necessary oxygen and nutrients aren’t scarce.

When the offspring settle out of the plankton, they might be in totally new environments: this gives the young a chance to survive across a wide area. The main disadvantage of this method is that the fish must go through a larval stage before they transform into adults. In this larval stage, they are very vulnerable while they try to find food and avoid predators. Also, they may not find a suitable environment when they settle out of the plankton column. The survival rate for individual eggs is very low, so the parent has to produce millions of eggs.

Egg depositors

These fish either lay eggs on a flat surface, like a stone or plant leaf or may even place them individually among fine leaved plants. The parents usually form pairs and guard the eggs and fry (young fish) from all danger. The Cichlids such as Koi are the best known species for this. Some Catfish and Rainbowfish are also egg depositors.

Nest Builders

Many fish species build nests. These might be a simple pit dug into gravel (trout do this) or an elaborate bubble nest. When they are ready to spawn, the fish may construct a nest by blowing bubbles, and they often use vegetation to anchor the nest. The male will keep the nest intact and keep a close eye on the eggs. The Gouramis, Anabantids and some catfish are the most common of this type.

Mouthbreeders

These are particularly odd, since eggs are fertilised externally, but raised internally.

The females usually lay their eggs on a flat surface where they are then fertilised by the male. After fertilisation the female picks up the eggs and incubates them in her mouth. Broods tend to be small, since by the time the fry are released by their mothers they are well formed and suffer minimal losses. The best known mouthbreeders are the African lake Cichlids.

Egg buriers

The annual Killifish reproduce in this way. As the pools they live in dry out, the fish spawn, pressing their eggs into the mud. The pools eventually dry out completely, killing the adults, but the eggs remain safe in the dried mud. When it rains and the pool refills the eggs can hatch and the cycle is repeated. Killifish eggs can survive for many years in dried out mud.

The young grow inside the mother, but are not nourished directly by her. In general, the young are advanced at birth -- most sharks and rays reproduce using this method -- but they may be born in a larval form like some scorpeaniforms such as rockfish.

The young grow inside the mother and are nourished by her directly with a placenta, like mammals. This method is rare in fish, but can be found in some sharks and surf perches. The broods produced by viviparous bearers are small and the fry are well developed when born.

In both Ovoviviparity and Viviparity the male uses a clasper, and perhaps teeth, to hold the female while he fertilises her, and the sperm passes down the clasper to gain entry to the female’s body. These methods of reproduction have the advantage that the eggs are much less vulnerable to predators when carried within the mother, and the young are born fully advanced and ready to deal with their environment as miniature adults. However, the adult must supply nutrients to its offspring and can only produce a few eggs at a time. The young can only be born into the environment that their parents were in, and if this environment is deteriorating and can’t support them, they must suffer it, and survive as best they can. This is often the case with established spawning grounds.

PARENTAL CARE

It is very rare to find fish giving much parental care, since most are broadcast spawners, but there are a few instances of adult fish showing a bit of TLC to their young. Male gobies, for instance, guard the eggs in a nest from spawning until the young are born, and the male yellowhead jawfish guards the eggs by holding them in his mouth.